Interfacial Stabilization of High-Voltage Cathodes in Lithium-Ion Batteries via Binder Design

Abstract

As global warming accelerates in the 21st century, measures to regulate carbon emissions are being actively discussed for sustainable development. In this trend, as the era of full-scale commercialization of electric vehicles begins, lithium-ion batteries (LIBs) and next-generation batteries are receiving tremendous attention. Currently, research is being conducted to increase energy density of the electrode with the goal of improving the mileage, and in the cathode part, high-voltage cathode materials such as layered and spinel structures are considered strong candidates to meet the required demand. However, when the cathode material is driven under a high cut-off voltage condition, the cathode-electrolyte interface (CEI) becomes very unstable, which leads to surface degradation such as electrolyte decomposition. Various attempts have been made to solve this problem, but in this work, the binder, which was mainly considered as an adhesive between the electrode components, was newly provided with the interfacial stabilization function of the high-voltage cathodes through the multifaceted design of the polymer structure and functional groups. The existing commercial PVDF binder has a relatively weak van der Waals interaction with metal oxide of the cathode. Therefore, the binder coverage on the cathode surface is insufficient so that the naïve cathode surface is easily exposed to the electrolyte. However, in this study, we developed a binder capable of inducing good coverage based on strong interactions such as hydrogen bonding and ion-dipole interaction to alleviate various surface degradations and to form a stable CEI. In chapter 1, spandex (SPDX) as highly elastic binder is applied for nickel-rich LiNi0.8Co0.1Mn0.1O2 (NCM811) cathodes in LIBs. The nickel-rich layered NCM811 is predominantly used for LIBs intended for electric vehicles owing to their high specific capacities and minimal use of high-cost cobalt. The intrinsic drawbacks of NCM811 with regard to cycle life and safety have largely been addressed by doping with foreign atoms and by applying surface coating. Here, we report that a highly elastic binder, namely SPDX, can overcome the problems of nickel-rich layered cathode materials and improve their electrochemical properties drastically. The high elasticity of spandex allows it to uniformly coat NCM811 particles via shear force during slurry mixing to protect the particles from undesired interfacial reactions during cycling. The uniform coating of spandex, together with its hydrogen bonding interaction with metal oxide of NCM811, leads to enhanced particle-to-particle interaction, which has multiple advantages, such as high loading capability, superior rate and cycling performance, and low binder content. This study highlights the promise of elastic binders to meet the ever-challenging criteria with respect to nickel-rich cathode materials in cells targeting electric vehicles. In chapter 2, λ-carrageenan (CRN) as a sacrificial binder is applied for 5 V LiNi0.5Mn1.5O4 (LNMO) cathodes in LIBs. The spinel LNMO is unique among the cathode materials used in LIBs as it can operate at the highest potential (~4.7 V vs. Li/Li+). However, high-voltage operation is a double-edged sword for cathodes: good for high energy density but unfavorable for stable cycling. Specifically, the high-voltage operation in turn decomposes the electrolyte at the CEI, impeding the rate and cycling performance and limiting the widespread industrial adoption of LNMO. This work introduces CRN as a binder for LNMO cathodes to overcome the two challenges these cathodes present during high-voltage operation: cycling stability and high-rate performance. The CRN binder provides good coverage of the LNMO particles via hydrogen and ion-dipole interaction to protect the LNMO surface and thus warrants stable cycling. Moreover, the sulfate group of CRN is decomposed to produce LiSOxF at the CEI, which supports Li-ion conduction and protects the interface from indiscriminate side reactions of the electrolyte to enhance the rate performance. Thus, the CRN binder is sacrificial. The concept of a sacrificial binder could be expanded to other emerging electrodes that are detrimentally affected by the oxidative/reductive decomposition of the electrolyte, highlighting the possibility that binders can play a more active role via their chemical functionality.

21세기에 접어들면서 지구온난화가 가속화됨에 따라, 지속 가능한 발전을 위해 탄소 배출에 대한 규제 방안이 활발하게 논의되고 있다. 이에 대한 일환으로 전기자동차의 상용화가 본격적으로 시작되면서 리튬이차전지 및 차세대전지에 대한 관심이 급증하고 있다. 현재, 주행거리 향상을 목표로 전극의 에너지밀도를 증가시키기 위해 연구가 진행되고 있는데, 양극 파트에서는 층상계, 스피넬 구조 등의 고전압 양극재가 요구되는 수요를 충족할 유력한 후보로 여겨진다. 하지만, 해당 양극재를 높은 cut-off voltage 조건에서 구동하면 양극-전해질 계면이 매우 불안정해져 전해질 분해 등의 표면 열화가 발생한다. 이를 해결하기 위한 다양한 시도가 존재하나, 본 연구에서는 주로 전극구성물질 간의 접착제로서의 역할만을 고수해왔던 바인더에 고분자 구조 및 작용기의 다각적 설계를 통해 고전압 양극재의 계면 안정화 기능을 새로이 부여하고자 하였다. 기존 상용화된 PVDF 바인더는 양극재의 metal oxide와는 비교적 약한 van der Waals interaction을 기반으로 결합한다. 따라서, 양극재 표면에 대한 coverage가 부족하여 naïve한 양극재 표면을 전해질에 많이 노출시킨다. 하지만, 본 연구에서는 양극재의 metal oxide와의 hydrogen bonding, ion-dipole interaction 등의 강한 interaction을 기반으로 우수한 coverage를 유도할 수 있는 바인더를 개발하여 각종 표면 부반응을 완화하고 안정적인 CEI를 형성할 수 있도록 시도하였다.